Implantable cardiac devices are well known in the art. They may take the form of implantable cardiac defibrillators or cardioverters (ICDs) which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered as a pacing system. The pacing system is comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pacemaker to the heart. A lead may provide both unipolar and bipolar pacing and/or sensing electrode configurations.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Implantable cardiac defibrillators are encapsulated in a conductive housing or enclosure. They are generally implanted in a pectoral region of a patient and also electrically connected to the heart with one or more electrode carrying leads implanted in the heart. An arrhythmia detector detects accelerated arrhythmias, such as ventricular tachycardia (VT) or ventricular fibrillation (VF). When such an accelerated arrhythmia is detected, a pulse generator delivers electrical therapy to the patient's heart. A therapy for tachycardia may be anti-tachycardia pacing and a therapy for fibrillation may be a defibrillation shock. Such therapies are well known.
The devices discussed thus far are fully implantable. They are fully implantable because the device enclosures are placed beneath the skin of the patient and the electrodes of the leads are positioned within the heart.
Subcutaneous cardiac stimulation devices and systems are also known in the art. These devices may also be implanted beneath the skin of a patient external to the heart. However, in these systems, the electrodes are not implanted within the heart. Rather, the electrodes are still placed beneath the skin of the patient but external to the heart.
There is a growing role for extravascular and extracardiac ICD implantation of subcutaneous ICDs. These devices are generally easier to implant than fully implantable devices and systems. Subcutaneous devices may be implanted in centers/sites that lack fluoroscopic capability. They should also be less expensive than their fully implantable counterparts.
Unfortunately, diagnosing VT/VF from subcutaneous electrodes spatially removed from the heart is challenging due to interference from non-cardiac signals, such as skeletal myopotentials, motion artifact, etc. Since treatment of VF requires high voltage therapy, which can be extremely painful for the patient, VF detection should be thoroughly verified before making a final diagnosis. Detection however must be sufficiently robust to adapt to a cardiac signal that may include interference not seen in traditional defibrillators having electrodes within the heart.
Ventricular fibrillation (VF) detection from remote sensing electrodes can be accomplished using two separate criteria, as, for example, disclosed in U.S. Pat. No. 7,403,813, filed Nov. 24, 2004, entitled SYSTEMS AND METHODS FOR DETECTION OF VT AND VF FROM REMOTE SENSING ELECTRODES, which application is fully hereby incorporated herein by reference. VF can be diagnosed when either no activity is being detected or when the activity detected has very high rates. The former case applies when the sensed signal amplitude suddenly drops at VF initiation, preventing a detector from sensing the signal. The latter applies when the signal amplitude does not drop, but the rate increases. In many cases the amplitude of the signal varies significantly, complicating the detection.
Hence, the present invention is generally concerned with thoroughly verifying arrhythmia detection before delivering therapy and appropriately compensating for abrupt changes in detected cardiac rate.